1. Field of the Invention
The present invention pertains to electronic inventorying of multiple objects and, more particularly, to a method of inventorying nested and remote objects utilizing a multi-level or chained radio frequency identification tag system.
2. Description of the Related Art
Accurate inventorying of bulk articles is vital to many aspects of business and governmental functions. Tracking the shipment and distribution of goods provides verification and accurate accounting, which enables effective management of resources, income, and expenses. It also enables monitoring the whereabouts of goods and equipment, such as military equipment, files containing sensitive information, and the like that are critical to local and national security.
Bulk articles stored in containers can be difficult to identify and track. Because they are not visible when stored in the container, bulk articles are not available for visual inspection and verification, and changes in their number and condition are not easily monitored. Doing so requires manual labor and breaking of the security of the container, which can result in compromising the condition of the goods and in delay of the shipment and distribution of the goods.
Bar codes are ineffective in monitoring bulk goods stored in containers because reading of the bar codes requires access to each object or its packaging. While X-ray is possible, it is not available for every type of material. In addition, X-rays do not provide a clear view of each item, and this requires an operator to count each item. Magnetic strips, unlike bar codes, are capable of being read and written to for continual updating. However, like bar codes, they are also not feasible for use with closed containers because, like bar codes, they must be accessible to an electronic strip reader.
Radio frequency identification (RFID) is another system used for tracking and identifying objects. A key feature of the RFID system is an information-encoded tag that responds to an interrogation signal from an interrogator. Generally, the tag is configured to return the interrogation signal via backscatter reflection. The reflected signal is modulated in accordance with the information stored in the tag.
As shown in FIG. 1, a basic RFID system 10 includes two components: an interrogator or reader 12 and a transponder (commonly called the RF tag) 14. The interrogator 12 and RF tag 14 include respective antennas 16, 18. In operation, the interrogator 12 transmits through its antenna 16 a radio frequency interrogation signal 20 to the antenna 18 of the RF tag 14. In response to receiving the interrogation signal 20, the RF tag 14 produces a backscatter modulated response signal 22 that is reflected back to the interrogator 12 through the tag antenna 18. This process is known as modulated backscatter.
The substantial advantage of RFID systems is the non-contact, non-line-of-sight capability of the technology. The interrogator 12 emits the interrogation signal 20 with a range from 1 inch to 100 feet or more, depending upon the power output and the radio frequency used. Tags can be read through a variety of medium, such as fog, ice, paint, dirt, odors, and other substances, including visually and environmentally challenging conditions where bar codes or other optically read technologies would be useless. RF tags can also be read at remarkable speeds, in most cases responding in less than 100 milliseconds.
A typical RF tag system 10 will contain a number of RF tags 14 and a single interrogator 12. The three main categories of RF tags are beam-powered passive tags, battery powered semi-passive tags, and active tags. Each operates in fundamentally different ways.
The beam-powered RF tag is often referred to as a passive device because it derives the energy needed for its operation from the interrogation signal beamed at it. The tag rectifies the field and changes the reflective characteristics of the tag itself, creating a change in reflectivity that is seen at the interrogator. A battery powered semi-passive RFID tag operates in a similar fashion, modulating its RF cross-section in order to reflect a delta to the interrogator to develop a communication link. Here, the battery is the source of the tag's operational power. Finally, in the active RF tag, a transmitter is used to create its own radio frequency energy powered by the battery.
Conventional continuous wave backscatter RF tag systems utilizing passive RF tags require adequate power from the interrogation signal 20 to power the internal circuitry in the RF tag 14 used to amplitude-modulate the response signal 22 back to the interrogator. While this is successful for tags that are located in close proximity to an interrogator, for example, less than three meters, this may be insufficient range for some applications, for example, which require greater than 100 meters.
There is a need for an RF tag system that can monitor the condition of goods stored in bulk containers and nested within other goods and containers. This need includes the ability to read tags that are out of RF range of the reader.